Pumps have been used for centuries for moving fluids, such as liquids, gases, slurries, etc., from a first location to a second location. Pumps can also operate as compressors or as vacuum pumps when the second location is a closed system, such as a pressure vessel. Pumps have wide application. One application for which pumps are employed is a pressure controller, which maintains and controls fluid pressure in a device. Generally described, pressure controllers are capable of maintaining or adjusting system pressure levels of an associated system by controlling the amount of fluid to be added thereto and/or removed therefrom.
Some currently available pressure controllers employ pumps to generate pressure. Such pumps utilize check valves of the ball/spring or poppet/spring type to preventing fluid flow in the reverse direction. Check valves of the ball/spring type include a ball that is held against a valve seat by a force from a spring thereby sealing the valve closed. To open the check valve, a pressure force greater than the spring force must be applied to move the ball away from the seat. In that regard, the pressure differential required to open the valve, sometimes referred to as the crack or cracking pressure, limits the lowest absolute pressure achievable in the device to be controlled. That is, the device to be controlled typically cannot be controlled to an absolute pressure level that is at or below the pressure required to open the valve.
In general, the rate of flow pumped by piston pumps utilizing check valves is controllable by varying the cycle rate of the piston. This limits the range of flow rates that may be generated. Pressure controllers, however, may need large flow rates to change pressure quickly and at other times may need small flow rates to maintain a stable reference pressure. Because of these needs, some pressure controllers use piston-pumps with check-valves to generate the large flow rates and additional control valves to generate small flow rates. The addition of the control valves, however, increases cost, size and weight of the pressure controller.
Furthermore, the device providing precise metering of fluids, e.g. the control valves, is separate from the device generating pressure, e.g. the piston-pump, and thus complicates control of pressure and/or flow rate. For instance, referring to a typical application of a pressure controller, wherein the pressure in a device to be controlled is currently low and is to be increased to a higher pressure and then held there precisely, initially the control valve is opened completely, and the pump is cycled at full speed. When the desired pressure is near, the pump may be slowed, and the control valve partially closed. If the pump is slowed too much, however, the pressure between the control valve and the pump will drop making it difficult to maintain flow through the control valve. If the flow is higher than the desired flow through the control valve, pressure upstream of the control valve will increase to the maximum pressure of the pump. This consumes excessive power and increases the required sensitivity of the control valve.